1. Field of the Invention
The present invention relates to a diversity receiver provided with at least two receivers.
2. Prior Art
There have been the diversity receiving system in which a receiver is fed the strongest signal of radio frequency signals supplied from a plurality of antennas so that the receiver can always deliver an audio output of a good quality. Meanwhile, a function has been proposed where the diversity receiver has two receivers so that various controls over the receivers can be performed with the audio output uninterrupted. One such function is the BSM (Best Stations Memory) where broadcasting stations are searched across the receiving frequency band of the radio receiver for signal strengths above a predetermined value so that several frequencies are stored into a memory in the order of signal strength and they are recalled later by means of a preset button. Another function is the Network-Follow, one of the services in the RDS broadcast which has been put in practical use in Europe. The RDS data of the station currently being received has the AF data, which are the frequency data of the other stations that are broadcasting the same program as the currently tuned station. The AF data is decoded and is stored into the memory residing within the receiver. The stored AF data is read out to receive an alternative station in the same network when the signal strength of the currently tuned station decreases below a predetermined level.
FIG. 3 shows a prior art diversity receiver provided with two receivers for effective use of the aforementioned functions. The receivers 6 and 7 are adapted to receive different frequencies. A microprocessor 8 supplies a command signal to both a first and second receivers 6 and 7 when the BSM function or the RDS network-follow function is required. An antenna 1 and an antenna 2 are mounted at different locations on a vehicle, and feed a radio frequency signal to an antenna switch circuit 12 which in turn selectively supplies the radio frequency signal to the respective receivers 6 and 7 under control of an antenna switch controller 5.
FIG. 4A illustrates the changes with time of the radio signals supplied to the on-vehicle receiver 6 running in a street. A signal S1 is outputted from the antenna 1 such as a rod antenna positioned at the front of the vehicle and a signal S2 from the antenna 2 such as an indoor antenna mounted on the rear window of the vehicle. While the vehicle is running, the output of the respective antennas exhibit substantially periodic changes in magnitude as shown in FIG. 4A due largely to multipath effect of the radio frequency signal. These signals differ from each other in time position of their peaks and valleys due to their different locations on the vehicle. The distances between peaks or between valleys account for the frequency of the signal.
Assuming that the signal from the antenna 1 has been selected by the antenna switch 12, the antenna switch controller 5 receives from the receiver 6 a field intensity signal indicative of the signal strength of the output from the antenna 1; the average value of the field intensity signal at an input terminal Sin, and a time varying component, i.e. noise component of the field intensity signal at an input terminal Tin through a capacitor C. The magnitude of the time varying component increases with decreasing signal strength from the antenna 1, i.e., the radio frequency signal becomes weak and noisy. When the time varying component exceeds a predetermined value, the antenna switch controller 5 holds across a hold capacitor CLH-1 the average value of the field strength signal supplied to the Sin. Then, the antenna switch controller 5 drives the antenna switch 12 to direct the radio frequency signal to the receiver 6 so as to receiver a field intensity signal indicative of the signal strength of the output from the antenna 1. Again, when the time varying component exceeds a predetermined value, the antenna switch controller 5 holds across a hold capacitor CLH-2 the average value of the field strength signal supplied to the Sin. The controller 5 compares the voltage across the capacitor CLH-1 with that across the capacitor CLH-2 to determine a capacitor with a higher voltage. The controller 5 drives the antenna switch 12 to select an antenna that results the higher voltage across the capacitor.
For example, the antenna 1 is assumed to have been selected. When the field strength signal decreases below the voltage across the hold capacitor CLH-1, the antenna switch 12 is shifted into the position of antenna 2. Then, when the field strength signal decreases below the voltage across the hold capacitor CLH-2, the antenna switch 12 is shifted again into the position of antenna 1.
FIG. 4B illustrates an antenna selecting signal outputted from the antenna switch controller 5 and FIG. 4A shows in a solid line the time varying magnitude of the signal S0 actually supplied into the receiver 6. The operation of the antenna switch can be controlled by a variety of methods well known in the art such that the signal S0 is fed to the receiver 6.
Let us consider that the network follow function is to be performed while the vehicle is running. FIG. 4C shows the change with time of field intensity signals S1' and S2' supplied to the receiver 7 while the vehicle is running. The antennas 1 and 2 are selected alternatively with a timing corresponding to the frequency being received by the receiver 6. There is no serious problem if the receivers 6 and 7 are to receive frequencies close to each other. However, the system suffers a serious problem if the two receivers are to receive frequencies relatively far from each other. FIG. 4C shows in a solid line the signal actually supplied to the receiver 7 when the receiver 7 receives a frequency quite different from that received by the receiver 6. In the figure, the input to the receiver 7 varies randomly with large changes in magnitude. This causes the demodulated output of the receiver 7 to be deteriorated seriously, failing to perform the network follow function.